Capacitive microelectromechanical system (MEMS) microphones are becoming increasingly important in various fields of application. This is essentially due to the miniaturized design of such components and the possibility for integrating additional functionalities at very low manufacturing costs. The integration of signal processing components such as filters and components for noise suppression, as well as components for generating a digital microphone signal, is particularly advantageous. Another advantage of MEMS microphones is their high temperature stability, which allows them to be installed in so-called “reflow solder processes,” for example.
The microphone capacitor is generally composed of a diaphragm which is deflected by acoustic pressure and which acts as a movable electrode, and an acoustically inactive stationary counter electrode. The acoustic pressure causes the distance between the diaphragm and the counter electrode to change, and also results in a change in capacitance of the microphone capacitor. These very small changes in capacitance in the AF range must be converted into a usable electrical signal.
One concept frequently implemented in practice is based on charging the microphone capacitor with a direct-current voltage via a high-impedance charging resistor. Changes in capacitance of the microphone capacitor are then detected as fluctuations in the output voltage, which is amplified via an impedance converter. This may be a JFET, for example, which converts the high impedance of the microphone in the range of Gohm into a relatively low output impedance in the range of several 100 ohms without altering the output voltage itself. Instead of a JFET, an operational amplifier circuit may be used which supplies a low output impedance. In contrast to the JFET, in this case the amplifying factor may be adapted to the particular microphone requirements. This concept has proven to be problematic in several respects:
The digital circuit elements together with the analog signal processing components are not easily implemented in CMOS technology due to the occurrence of electrical noise, so-called “1/f noise.” The JFET technology, which requires low noise, cannot be achieved within the scope of standard CMOS processes, and instead requires relatively costly specialized processes.
Electrostatic forces of attraction are present between the diaphragm and the counter electrode due to the direct-current voltage applied to the microphone capacitor during operation of the microphone. These electrostatic forces are critical in particular in overload situations, since they promote continuous adherence of the diaphragm to the counter electrode, resulting in a breakdown of the microphone function. To detach the diaphragm from the counter electrode, it is generally necessary to completely discharge the microphone capacitor. In practice, mechanical measures such as a relatively stiff diaphragm suspension, a relatively large distance between the diaphragm and the counter electrode, or mechanical stops, for example, have been attempted in order to avoid such electrostatic collapse. However, these measures usually have an adverse effect on the sensitivity of the microphone, or are very complicated from the point of view of the manufacturing process.
Lastly, it is noted that a relatively high direct-current voltage in the range of 10 volts and greater must be applied to the microphone capacitor to achieve a sufficiently high signal-to-noise ratio (SNR). However, a charging voltage in this range often requires a comparatively large distance between the diaphragm and the counter electrode in the range of >>2 μm or a very stiff diaphragm design, on the one hand to avoid electrostatic collapse, and on the other hand to provide a sufficiently large deflection range for the diaphragm. Such large distances or very stiff diaphragm designs are not easily provided using standard surface micromechanical methods. In addition, such high charging voltages in overload situations, with contact from the diaphragm and the counter electrode, result not only in adherence of the diaphragm to the counter electrode, but also irreversible melting of the contact surfaces due to current flow. In practice, attempts have been made to prevent this with the aid of insulating layers. However, these increase the complexity of the manufacturing process, and therefore ultimately increase the costs for such a microphone component.
German patent application 10 2009 000950.7 A1 proposes a microphone component of the type mentioned at the outset, which may be operated at a relatively low voltage level and still has a comparatively high sensitivity and SNR.
The concept on which this MEMS microphone is based provides that a high-frequency sampling signal is applied to the microphone capacitor, and the inverted clock signal is applied to an adjustable but acoustically inactive compensation capacitor. The sum of the current flow through the microphone capacitor and the current flow through the compensation capacitor is integrated with the aid of an integrating operational amplifier. The output signal of the integrating operational amplifier is then demodulated with the aid of a demodulator which is synchronized with the clock signal. Lastly, a microphone signal which corresponds to the changes in capacitance of the microphone capacitor is obtained by low-pass filtering of the demodulated signal.
In the case of DE 10 2009 000950.7 A1, the high-frequency sampling signal is a symmetrical clock signal in the form of a square wave voltage having a 1:1 ratio of the clock times.
The adjustable compensation capacitor is used for compensating for the current flow which flows through the microphone capacitor and is not due to acoustic effects. The aim is to operate the input of the subsequent charge amplifier close to its neutral voltage. Ideally, the compensation capacitor is adjusted corresponding to the quiescent capacitance of the microphone capacitor. Since the inverted clock signal is present at the compensation capacitor while the microphone capacitor is supplied with the clock signal, the operational amplifier integrates only the component of the current flow through the microphone capacitor which is due to the acoustically related changes in capacitance of the microphone capacitor, and therefore, the deviations from symmetry. Based on the output signal of the integrating operational amplifier, a microphone signal which reflects these changes in capacitance may then be obtained relatively easily, namely by synchronized demodulation and low-pass filtering.
At a voltage level of the high-frequency clock signal of less than 2 volts, this type of signal detection provides acceptable sensitivity, i.e., a sufficiently high SNR. This is also advantageous in particular in overload situations. Namely, for voltages in this range, contact between the diaphragm and the counter electrode does not result in melting of the contact surfaces, and therefore also does not result in destruction of the microphone structure. For this reason, mechanical overload protection in the form of electrically insulating stops may be dispensed with here. In addition, the micromechanical structure of this microphone component as well as the circuitry components thereof for signal detection may be produced using standard processes of MEMS technology or CMOS technology, and therefore in a very cost-effective manner.
Due to the small installation size, the relatively high sensitivity, and the low manufacturing costs, the microphone component discussed in DE 10 2009 000950.7 A1 is very well suited for use in mass-produced portable devices such as mobile telephones, for example. For such applications the power consumption of the microphone component must be kept as low as possible. A primary power consumer of the microphone component discussed in DE 10 2009 000950.7 A1 is the integrating operational amplifier, which is operated as a low-noise charge amplifier.